High intensity discharge (HID) lamps such as metal halide (MH) lamps, high pressure sodium (HPS) lamps and high pressure mercury vapor lamps have increasingly gained acceptance over incandescent and fluorescent lamps for commercial and industrial applications. HID lamps are more efficient and more cost effective than incandescent and fluorescent lamps for illuminating large open spaces such as construction sites, stadiums, parking lots, warehouses, and so on, as well as for illumination along roadways. Commercial HID lighting installations generally employ luminaires which are complete lighting units, each of which comprises a ballast and its housing, a lamp socket, and a lamp.
Additional savings with regard to further reducing energy consumption can be achieved by replacing an HID lamp of a particular type and wattage with a lower wattage HID lamp. For example, an entire luminaire having a conventional fluorescent lamp and its ballast can be replaced with a luminaire having a lower wattage HPS lamp and ballast therefor. Replacing the entire luminaire, however, is costly since all of the luminaire parts are being replaced.
As an alternative, a substitute lamp having a lower rated lamp wattage can be used with the existing ballast in a luminaire. This approach, however, is disadvantageous. Although a reduction in lamp wattage can result in an energy savings, the substitute lamp and the ballast are not matched so as to yield the most efficient performance.
A consequence of a mismatched lamp and ballast can be an increase in lamp current. The increased lamp current which occurs upon the substitution of a lamp having a lower rated lamp voltage into an existing luminaire is addressed in U.S. Pat. Nos. 5,606,222, to Cottaar et al. 5,606,222, to Cottaar et al, relates to a current-reducing device for reducing current through a lamp and ballast to reduce system wattage in a gas discharge lamp lighting system. The current-reducing device is described as a capacitor connected in parallel with the discharge lamp in a system having a lead-type ballast (e.g., a constant-wattage auto transformer (CWA)). The impedance of the parallel capacitor is selected to be between ten and twenty times the impedance of the lamp such that the capacitor is configured to take a substantial amount of current. This parallel arrangement is disadvantageous because it merely diverts energy that would normally flow through the lamp when no such arrangement is used. The reduced current through the lamp is unacceptable because it deteriorates the waveform provided to the lamp, thereby decreasing the operating life of the lamp. For example, the current crest factor increases, among other undesirable waveform changes, and prevents the lamp from operating optimally and in accordance with the lamp characteristics with which the lamp was designed to operate, including but not limited to open-voltage and sustaining voltage requirements, ignition and starting current requirements, lamp regulation requirements, and so on. Accordingly, a need exists for an apparatus and method to reduce system wattage in retrofit applications for gas discharge lamps (i.e., substitution of a typically lower wattage lamp using an existing ballast coil and core) which does not significantly shorten the operating life of the substituted, lower wattage lamp.